Respiratory syncytial virus is an important cause of severe lower respiratory tract illness in young children. Those at particular risk for serious RSV illness are children with bronchopulmonary dysplasia, the chronic lung disease of prematurity with respiratory distress and an oxygen requirement lasting at least 30 days, and those with complex congenital heart disease. We plan to test the hypothesis that PFP-2 vaccine is safe, immunogenic and will prevent or significantly ameliorate serious RSV illness in high-risk children.